Efficient light coupling between an optical fiber and a silicon waveguide is highly desired for silicon based photonic device and circuit applications. Due to the high refractive index contrast of silicon waveguide systems, obtaining good fiber-silicon waveguide coupling is very challenging particularly for small silicon rib waveguides.
Often is the case that an optical device includes a fiber or waveguide that is intended to be coupled to another waveguide having a significantly larger/smaller cross-sectional size. For example, a planar lightwave circuit (PLC) can have a waveguide on the order of four microns in height to be coupled to an optical fiber with a diameter of about ten microns. One way to couple a port of a relatively large waveguide to a port of a significantly smaller waveguide is by forming a tapered waveguide structure to couple the two waveguides.
In one type of taper, similar to that shown in U.S. Pat. No. 6,108,478 to Harpin et al., the taper at one end has a height or diameter of about the same size as a larger waveguide to which it is to be coupled. At the other end, the taper typically comes to a point. The sides of the taper are typically straight so that the taper has a wedge-like shape, with the wider part of the taper being at the end of the waveguide. This end of the taper is used to couple the taper to the larger waveguide. The interior end of the taper serves as a termination, which along with the narrowing shape of the taper helps force light to propagate from the wide end of the taper to the smaller waveguide (or from the smaller waveguide to the wide end of the taper).
FIG. 1 shows a tapered rib waveguide, sometimes referred to as a “cheese wedge” taper waveguide 100 similar to that shown in Harpin et al, mentioned above. The waveguide 100 may be formed on a silicon-on-insulator (SOI) substrate comprising an insulation layer 102 and a silicon layer 104. The waveguide 100 generally comprises a tapered section 106 and a final waveguide or rib section 108, shown divided by illustrative line 111. The tapered section 106 comprises a lower taper 110 and an upper, generally wedge shaped taper 112. The upper taper 112 and lower taper 110 include an input facet 114 which may be integrally formed. The lower taper 110 gradually tapers down over length “L” to match the size of an output waveguide 116 in section 108. The upper taper 112 may taper to a point 118 to be generally wedge shaped. This type of waveguide taper 100 may be used to provide high coupling efficiency (coupling loss <1 dB/facet) between a standard fiber (with a modal diameter of ˜9 μm) coupled at the input facet 114 and silicon waveguide 116 with a width or height of ˜4–5 μm.
As the refractive index contrast is larger for silicon waveguides as compared to optical fibers, a larger taper input facet 114 may be needed for better coupling. For example, as shown in FIG. 1, if a larger input facet 114 of 13×13 μm is called for, it may be difficult to obtain efficient coupling to waveguides 116 smaller than W×H=2.5×2.5 μm with a reasonable taper length (e.g., L=1–2 mm).
Referring to FIG. 2 there are shown modeling results for taper loss for a 2 mm long taper (L) for different final rib waveguide sizes (W) 116. The graph shows that the loss for the standard cheese wedge taper 100 increases with decreasing the final waveguide dimension. When the waveguide dimension W is smaller than 2.5×2.5 μm, the loss is larger than 1 dB/facet. Further, loss increases quickly with further decreasing the final waveguide 116 dimension. Since these small waveguides (1–2.5 μm) are often used for the high density silicon photonic integrated circuits and for better laser diode to silicon waveguide coupling, such losses may be unacceptable.